Interpretive Summary: Nest usurpation (also known as colony takeover) consists of a small swarm of African honey bees invading a European honey bee colony. The African queen in the swarm replaces the European queen in the invaded colony. Colony takeover has been suggested to play a role in the success of African bees in the New World. While nest usurpation has been previously documented in South America, it has not been reported to occur in the U.S. We monitored usurpation activity over a two-year period in an apiary with 76 five-frame nucleus colonies in Tucson, AZ. Each colony was headed by a "Golden Italian" European queen. We examined the colonies every 10-14 days to determine if the colony was thriving, weak or queenless. Weak colonies were requeened. We observed an average annual usurpation rate of 21.1 plus or minus 9.3%. The greatest usurpation activity occurred from October-December and coincided with the period of seasonal absconding by African bees in the Tucson region. Thriving and weak colonies experienced similar, low monthly usurpation rates. However, queenless colonies and those that had a caged queen or had been recently requeened were 2-8 times more likely to be invaded especially in the fall-winter. These results suggest that cues associated with compromised queen condition or diminished brood rearing could be used by usurpation is an important, but annually variable factor contributing to the spread of African bees in the southwestern U.S.

Technical Abstract:
We examined nest usurpation (colony takeover) by African honey bee swarms over a two-year period in an apiary that contained 76 European colonies maintained in Tucson, AZ. We observed a mean annual usurpation rate of 21%, with highest usurpation rates occurring from October-December. Queenless colonies, colonies that had a caged queen and those that had been recently re-queened were 2-8 times more likely to be invaded than were colonies that contained an actively laying queen. This trend was particularly pronounced in October-December. During those months the usurpation rates experienced by queenless and caged-queen colonies approached 20-50%. These results suggest that nest usurpation may be an important factor contributing to the spread of African bees and the loss of European honey bee matrilines in the southwestern U.S., and that queen condition may have a major influence on host colony susceptibility to usurpation.